Magnetic resonance imaging systems rely on the tendency of atomic nuclei possessing magnetic moments to align their spins with an external magnetic field. Because only nuclei with odd numbers of nucleons have a magnetic moment, only those nuclei can be detected and imaged using magnetic resonance. At present, hydrogen with one nucleon, a proton, in its nucleus is the element of choice for diagnostic tissue imaging.
The intensity of the magnetic resonance (MR) image depends upon the nuclear spin relaxation parameters denoted T.sub.1 and T.sub.2 in the region being imaged. T.sub.1 is the rate constant for signal recovery after the nuclear spins have been inverted by a radiofrequency pulse. T.sub.1 is called the spin lattice relaxation time.
T.sub.2 depends upon the dephasing of proton spins with each other as they precess in the magnetic field. T.sub.2 is called the spin-spin relaxation time.
Other factors influence the MR image. Such factors include concentration of nuclear spins, diffusion, and chemical shift.
Contrast media also have an influence on the MR image. Paramagnetic metals have been traditionally used as contrast media, but are not universally useful, and are costly. Other contrast agents useful in MR imaging of the gastrointestinal tract include superparamagnetic particles, clay minerals and perfluoro compounds.
A problem with these contrast agents is their inability to be used in certain MR imaging techniques such as lipid-suppressed T.sub.2 imaging. Lipid-suppressed T.sub.2 imaging involves the use of lipid signal suppression combined with a long T.sub.2 weighting in the same pulse sequence. Under such conditions, a compound with one or more strong and narrow proton nuclear magnetic resonance signals can be observed, provided that the T.sub.2 values of the nuclei giving rise to the lines are sufficiently long, and the chemical shift of the line(s) is significantly different from that of the lipid methylene protons in fat.
It would therefore be useful to have a contrast agent that is inexpensive and provides an image of organs and organ systems such as the gastrointestinal tract that would take advantage of the usefulness of lipid-suppressed T.sub.2 weighted imaging.